The present invention relates to a threaded fastener for joining together large component parts of a machine and, more particularly, to a multi-piece construction of such a fastener for assembly onto external threads of a shaft and using a stress generator, preferably jack bolts, to impose a pre-stressing force on the threaded connection without applying torque to the threaded connection.
Heavy-duty forging presses are used, for example, in the metal working industry to shape and refine the metallurgical grain structure of a metal casting, usually a steel workpiece, by forging (pressing) the steel workpiece in a highly heated condition between dies. A forging press of this type is typically required to develop 5,000, or more tons of pressure between the dies and consists of massive parts essentially including a hydraulically driven ram in cylinder installed in a stationary upper crosshead. One or two pairs of tie rods sometimes called steel columns interconnect and anchor the upper crosshead to a large base. These parts are designed to be very heavy duty and thus usually very large so as to sustain the forces developed by the forging operation. The hydraulically driven ram is operated to move a crosshead carrying the upper forging die against the heated steel workpiece. The workpiece is manipulated between strokes of the ram for the forging operation into strategic positions resting on the stationary die located on the large stationary base to which the steel columns are attached. Other hydraulically operated piston and cylinder assemblies lift the ram; the crosshead and the upper forging die after each stroke of the ram for the forging operation.
The steel columns are used in pairs to tie the stationary upper cross head to the large stationary base and must resist the pressure developed between the dies. The steel columns have threaded ends protruding from the crosshead and base to receive a threaded nut member. Each nut member is provided with a threaded bore, typically, of the order of between one foot and three feet in diameter and torque must be applied to pre-stress the threaded connection to prevent impact loading between mating threads. The magnitude of the pre-stressing force should be in excess of the working forces including impact loading occurring during the forging operations so as to prevent permanent deformation particularly where clearances occur between the mating threads of nuts on the threaded ends of the steel columns. The threads on the steel column and the fastener nuts are of such physical proportions that the use of mechanized handling equipment is necessary for the removal and installation of the fastener nuts to accomplish necessary disassembly and reassembly of the forging press for servicing operations. Encrusting with air born particulates on the exposed threads of the steel columns that extend beyond each nut can cause a wedged or jammed condition when unthreading of the mating threads. Another problem occurring when servicing an existing forging press is deformation of the thread on the nut and the thread on the steel column at the site of the initial load transfer area is particularly troublesome because the distortion of the nut thread distorts the thread pitch that must traverse the entire thread travel during the nut removal process. These conditions are frequently known as nut lock-up and galling conditions. Even when installing nuts on new press columns, the presence of metal debris such as chips or nonconforming threads or even standard handling procedures can cause galling to occur. The weight of the nut and large contact area between mating threads can be sufficient to cause galling. Also, when an existing nut member cannot be reinstalled because of damage, the thread tolerance allowed within the thread on the steel column when manufactured, sometimes 30, 50 or more years historically, is very imprecise as compared with modern day standard tolerances, and thus thread mating difficulties occur when installing a newly manufactured replacement nut member. Similar problems are encountered in the construction and maintenance of other heavy-duty equipment used in the metal working industry and other industries where the force of a very large magnitude demands the use of threaded fastening nut members having such large physical size and weights so as to require mechanized handling equipment for installation and removal.
Accordingly, it is an object of the present invention to provide a fastener for ease of assembling onto and tensioning large threaded columns, rods, like those used in heavy-duty machinery, particularly forging presses, without requiring heavy-duty mechanized equipment for installation including pre-stressing of mating threads and removal of the fastener.
It is another object of the present invention to provide a fastener construction useful in the installation operation with the aid of ordinary hand tools and eliminate nut lock-up and galling conditions known to occur in the use of large heavy duty fasteners used to resist large forces.